Servo-assisted steering systems of the above-mentioned type are described, for example, in German Published Patent Application No. 42 20 624. In these, the torque-dependent actuation of the servo actuator assisting the steering force is controlled by a hydraulic steering valve in the form of a rotary slide valve. For this purpose, the rotary slide of the rotary slide valve is firmly connected to an input element formed by the steering shaft so as to rotate with it, and the input element is connected so as to rotate with a limited rotational angle and dependent on the torque with respect to an output element connected to the steering mechanism via a torsion spring. The output element is in turn fixed against rotation to a control bush which is coaxial with the rotary slide and can rotate with respect to the latter. Depending on the torque-dependent rotation of the control bush with respect to the rotary slide, the result is different degrees of overlap between the corresponding connection cross sections of these, and thus different opening cross-sections in the feed and discharge of the servo actuator. The torque assistance via the torsion spring has speed-dependent torque assistance superimposed on it and, for this purpose, the control bush is acted on axially at the end by a reaction piston which, in relation to the input element and to the rotary slide, is supported firmly such that it cannot rotate but can be displaced axially and is spring loaded in the direction of the control bush. The mutually facing ends of the control bush and of the reaction piston have displacement surfaces which are inclined in opposite directions, between which there are rolling elements, so that, depending on the torque-dependent rotation of the control bush with respect the rotary slide or the input element, as a result of the rolling elements running onto the displacement surfaces, the result is a displacement of the reaction piston counter to the force of the latter in the direction of the spring acting on the control bush.
The chamber which accommodates the spring is designed as a pressure chamber and is to be connected to the high-pressure side of the hydraulic circuit such that the reaction piston, assisting the loading spring force, is pressure-loaded in the direction of the control bush, so that as the pressure increases, the assistance between input and output element becomes rotationally more stiff. The pressure supplied to the reaction chamber is controlled as a function of speed via an electro-hydraulic converter with an actuating magnet that acts on a hydraulic valve arrangement.
A steering valve of this type has an initial position at speeds around zero which corresponds to a position of maximum servo action, such as is desired for parking, for example. Starting from this position, the steering becomes rotationally stiffer with increasing speed as a result of the superimposed pressure loading of the reaction piston, in the same direction as he spring force, so that a higher input torque has to be applied for steering movements. German Published Patent Application No. 42 01 311 describes a servo-assisted steering system with the same basic structure, but the steering valve has an initial position which corresponds to a position of minimum servo action, such as is the intention for higher speed, for example, in order to avoid a woolly steering feel by an appropriately rotationally stiff steering system, and to obtain improved feedback from the roadway. The reaction piston is pressurized in the direction opposite to the spring force as a function of speed, specifically such that, at speeds around zero, the result is the intended high servo action, in which the steering torque input on the input side is assisted in a relatively rotationally soft manner because the reduced stiffening of the torsion spring via the spring-loaded reaction piston as a result of the pressurization.
Depending on the initial position, therefore, the pressurization of the reaction piston can be used to adjust the steering system in the hard direction with increasing speed, with the pressurization in the same direction as the spring force or, with pressurization in the opposite direction, to make the steering system rotationally softer with reducing speed, starting from its hard, rotationally stiff setting, as is desired for parking.
Irrespective of the respective constructional design in this regard, on the steering valve side, as a result of the rotary slide and the superimposed, speed-dependently operating actuating and control elements, a relatively long function chain with alternating influences is provided and, irrespective of the testing and setting of the individual functional elements, tolerances can occur which, in particular in the context of mass production, lead to scatter with respect to the respective valve characteristic curve, which also become noticeable in vehicles with the same equipment level and impart a different steering feel. Such mass-production scatter leads to complaints in the case of manufacturers of vehicles, and likewise in the case of end customers, in particular if these are sensed as deviations from the usual.
Since the checking of the steering in its entirety and in the overall assembly in the vehicle is barely possible with tolerable effort, but in particular access to the hydraulic elements cannot be implemented without dismantling, it is necessary to fall back on specific setting devices in testing the individual elements. With their aid, the electro-hydraulic converter is set by mechanical intervention to a converter characteristic curve which corresponds in the best possible manner to the set characteristic curve of the steering valve and which, therefore, as based on the effective coil current of the converter, results in a reaction pressure, i.e., a build-up of pressure in the reaction chamber partly bounded by the reaction piston, as a function of speed, which corresponds to the respectively intended servo action illustrated by the set valve characteristic curve.
A calibration of this type of the electro-hydraulic converter is influenced as such by the tolerances of the associated setting device and in the pairing with the hydraulic valve arranged downstream, in particular a rotary slide valve, has superimposed on it tolerances of the latter and those of a steering mechanism arranged downstream. Subsequent intervention possibilities, in the sense of readjustment by mechanical intervention in the hydraulic steering valve, are virtually not provided, and thus subsequent changes are at most possible with considerable effort.
It is an aspect of the present invention to provide for, e.g., with little effort, and, e.g., which may also be managed well in mass production, limiting the tolerances in relation to the set valve characteristic curve associated with the intended steering behavior. A further aspect of the present invention is to provide a testing method which may make it possible, e.g., with little effort and e.g., one which may also be managed well in mass production, of limiting the tolerances in relation to the set steering characteristic curve associated with an intended steering behavior and of permitting the set valve characteristic curve corresponding to this.